Authentication devices are useful in a wide variety of applications. These devices are often referred to as “smart cards” since they are usually in the form of cards and they have the ability of performing certain tasks, unlike any conventional cards such as ordinary debit cards or credit cards. When constructed as cards, authentication devices are generally built in accordance with the ISO 7816 standard.
An acoustic authentication device is a specialized kind of authentication device. Unlike other authentication devices, an acoustic authentication device does not require a specialized terminal to transmit information. The information is rather transmitted as a sound sequence to be received through acoustic coupling by an acoustic interface, such as a telephone handset, a microphone or any other suitable communication terminal. The sound sequence emitted by an acoustic authentication device is ultimately transmitted to a transaction system. The sound sequence is then analyzed to extract the data string containing the information and determine whether it is genuine or not. When a successful recognition is made, the device holder usually receives an access or another privilege, depending on the exact situation. For instance, the device holder may gain access to a computer system, a door, a long distance call, etc. Acoustic authentication devices have many advantages over non-acoustic authentication devices such as conventional smart cards. Among other things, they do not require a special and usually costly terminal. They can be used virtually anywhere a telephone or any other device equipped with a microphone is available.
When acoustic authentication devices are used, various factors can affect signal recognition at the transaction system. The success rate of correctly transmitting a sound sequence is generally limited by the quality of the terminal and by the transmission channels that form the communication medium. These elements may unfortunately affect the sound sequence and prevent the transaction system from obtaining a successful recognition even if it originates from a genuine acoustic authentication device. Among the possible problems that may affect transmitted sound sequences, there are:                interference produced by the transmission channels and which can be misinterpreted by signal processing tools at the receiving end;        ambient noise which induces a bad signal-to-noise ratio, especially when the devices operates with frequencies in the telephone band;        important variations in the signal amplitude, or even a loss of signal, caused by the movement of hand-held devices by its user, the probability of such movement occurring during transmission of the sound sequence being directly proportional to the length thereof;        communication interruptions (especially on cellular phones) which can lead to a loss of data;        frequency shifting due to the transmitting device constraints;        noise cancellation (or attenuation) on terminals like cellular phones that can affect signals like FSK;        variation over time of the frequencies generated by some microchips used in acoustic authentication devices; and        the variation in frequency response between piezoelectric elements used in various acoustic authentication devices.        
Another problem with acoustic authentication devices is concerned with the use of batteries therein. A self-powered acoustic authentication device uses one or more batteries to provide the electric power required for the microchip and other components to function. Since most of these devices are generally in the form of thin cards, they are provided with very small non-replaceable batteries embedded therein. The problem is that it is not always possible to obtain a constant voltage level from the battery or batteries. Typically, the output voltage level of a battery progressively decreases until the device cannot work anymore. A battery also looses some power even when the device is not used. The life expectancy is thus directly dependant on either the number of times the device is activated and on the time since the device was initially put in operation.
Almost all acoustic authentication devices are autonomous and rely on an internal clock to time their microchip and generate the sound sequences. When no internal crystal can be used for the clock because of size constraints, which is presently the case for devices built as ISO 7816 cards, the problem is that the internal clock frequency usually decreases when the supplied voltage decreases. Thus, when generating sound sequences, a frequency drift may appear over time, eventually making it more difficult to obtain high recognition rates during signal processing at the transaction systems.
Furthermore, it was difficult to provide a way, built on or into the device, to estimate the exact remaining battery life, without using additional components which increase the overall unitary cost of the device. Whenever a non-replaceable battery is empty in a device, the device holder must obtain a new device to resume normal activities. This may be problematic if it is not known in advance when a device will cease to work or if the frequency drift is too important to ensure a successful recognition by the transaction system.